The critical role of human growth hormone (hGH) in postnatal growth in humans is well recognised. Less obvious is the impact of this hormone on the regulation of lipid and carbohydrate metabolism, due to lack of detailed molecular studies.
It is well documented that the predominant form of hGH is a globular protein with a molecular weight of 22,000 daltons (22-KD) and consists of 191 amino acid residues in a single-chain, folded by 2 disulphide bonds with a small loop at the carboxyl terminus between residues 182 and 189. Recent crystallographic studies also show that the hGH molecule contains four anti-parallel .alpha.-helices which are arranged in a left-twisted, tightly-packed helical bundle.sup.1. The concept that there are discrete functional domains within the hGH molecule responsible for specific metabolic actions of the hormone is generally accepted. The amino-terminus has been identified as the functional domain responsible for the insulin-like actions of the hGH molecule.sup.2,3.
Recombinant DNA technology opens the way to the large-scale commercial production of human growth hormone, and the recombinant hGH appears to have equivalent biological efficacies and pharmacokinetic properties.sup.4,5. Current supply of this multiple-functional hormone no longer restricts the types and numbers of experimental therapies in humans and animals. The use of hGH for treatment of short stature in children and adults is well-established.sup.6. Therapeutic effects of hGH in female infertility have also been reported.sup.7,8. Treatment of human obesity with hGH has been advocated because of its remarkable effects on body composition with lipid metabolism.sup.9. However, the clinical applications of the intact hormone encounter a variety of problems. Evidence suggests that this multiple-functional hormone often simultaneously exerts in vivo, by various bioactive domains within the molecules, some adverse effects.sup.10.
Regulation of lipid metabolism by GH was first described in 1959 by Raben & Hollenberg.sup.11. The acute increase of plasma free fatty acids after GH administration was the major evidence for this metabolic function of the hormone. The regulatory role of the hormone in lipid metabolism was subsequently supported by the body composition studies of GH-deficient and GH-treated humans.sup.12,13 and pigs.sup.14,15. The findings of Gertner suggest that hGH is linked to adipose tissue distribution through a series of interactions known as the "GH-fat cycle".sup.16. However, the molecular events transpiring to these biochemical and physiological changes remained largely unknown. The metabolic effects of GH on adipose and other tissues in vivo are variable and complex, apparently consisting of at least two components, an early insulin-like effect followed by a later more profound anti-insulin effect.sup.17. The results of the latter effect may include both a stimulation of lipolysis and an inhibition of lipogenesis. The anti-lipogenic effect of hGH has been substantiated with the demonstrations of the decrease of the expression of glucose transporter GLUT 4 in adipocytes.sup.18, the inhibition of the activity of acetyl-CoA carboxylase in adipose tissues.sup.19,20 and the reduction of glucose incorporation into lipid in both isolated cells and tissues.sup.21,22.
In view of the multiple-functional effects of intact hGH and the problems encountered in clinical applications of the intact hormone, work leading to the resent invention has been directed to investigating whether hGH derivatives could be synthesised that retain the desired bioactivities and lack the unwanted side effects. In this work, structure-function studies of hGH have been carried out to elucidate the molecular mechanism of the metabolic actions of this multiple-functional hormone.
The structure-function studies of hGH with synthetic hormonal fragments have revealed that the carboxyl terminus of the hGH molecule appears to be the functional domain of the hormone for the regulation of lipid metabolism.sup.20,23,24,25 and it has now been shown that a synthetic peptide having a sequence based in the carboxyl terminal region reduces body weight gain and adipose tissue mass in a laboratory obese animal model.